/// <summary>
/// Creates a new LRItem with the given dot index from the given other item.
/// </summary>
/// <param name="dotIndex"></param>
/// <param name="otherItem"></param>
public LRItem(int dotIndex, LRItem <T> otherItem)
{
LeftHandSide = otherItem.LeftHandSide;
ProductionElements = otherItem.ProductionElements;
DotIndex = dotIndex;
LookaheadElement = otherItem.LookaheadElement;
}
/// <summary>
/// Gets a collection of Terminal elements that can appear after the element that is after the dot of the item.
/// </summary>
/// <example>
/// With Grammar:
/// S -> E
/// E -> T
/// E -> (E)
/// T -> n
/// T -> + T
/// T -> T + n
/// Follow(S -> •E) : {$}
/// Follow(T -> •+ T) : {'+', 'n'}
/// Follow(T -> •n, ')') : {')'}
/// </example>
/// <param name="nonTerminal"></param>
/// <returns></returns>
public IEnumerable <Terminal <T> > Follow(LRItem <T> item)
{
//Follow(item) is First(b) where item is:
//A -> a•Eb
GrammarElement <T> element = item.GetNextElement(1);
if (element != null)
{
//if(item.LookaheadElement != null && item.GetNextElement(2) == null)
//{
// //if it is, then include the lookahead of item in the follow set.
// return First(element).Concat(new[] { item.LookaheadElement });
//}
////If the element is not the last element in the production.
//else
//{
return(First(element));
//}
}
if (item.LookaheadElement == null)
{
return new[] { EndOfInputElement }
}
;
return(new[] { item.LookaheadElement });
}
/// <summary>
/// Returns whether the value(s) contained by the given other LRItem equals this object.
/// </summary>
/// <param name="other"></param>
/// <returns></returns>
public bool Equals(LRItem <T> other)
{
if (LookaheadElement != null &&
other.LookaheadElement != null)
{
return(LeftHandSide.Equals(other.LeftHandSide) && DotIndex == other.DotIndex && ProductionElements.SequenceEqual(other.ProductionElements) && LookaheadElement.Equals(other.LookaheadElement));
}
return(LeftHandSide.Equals(other.LeftHandSide) && DotIndex == other.DotIndex && ProductionElements.SequenceEqual(other.ProductionElements));
}
/// <summary>
/// Gets the LR(1) Closure of the given item.
/// </summary>
/// <param name="item"></param>
/// <returns></returns>
public IEnumerable <LRItem <T> > LR1Closure(LRItem <T> item)
{
if (closures.ContainsKey(item))
{
return(closures[item]);
}
IEnumerable <LRItem <T> > closure = Lr1Closure(item, new HashSet <LRItem <T> >());
closures.Add(item, closure);
return(closure);
}
/// <summary>
/// Creates the first state.
/// </summary>
/// <returns></returns>
public IEnumerable <LRItem <T> > CreateFirstState()
{
//get the starting item, S' -> S, $
var startingItem = new LRItem <T>(0, Productions[0], EndOfInputElement);
var firstState = new List <LRItem <T> >();
//add S' -> S, $ to the first state
firstState.Add(startingItem);
//get the rest of the first state
firstState.AddRange(Lr1Closure(startingItem, new HashSet <LRItem <T> >()));
return(firstState);
}
/// <summary>
/// Gets the LR(1) Closure of the given item, using currentItems to filter out duplicates.
/// </summary>
/// <param name="item"></param>
/// <param name="currentItems"></param>
/// <returns></returns>
private IEnumerable <LRItem <T> > Lr1Closure(LRItem <T> item, HashSet <LRItem <T> > currentItems)
{
//<LRItem<T>> items;
//items = new List<LRItem<T>>();
//add the current items
//if (currentItems != null)
//{
// items.AddRange(currentItems);
//}
GrammarElement <T> nextElement;
//if the next element is a non terminal
if ((nextElement = item.GetNextElement()) is NonTerminal <T> )
{
//get all of the productions whose LHS equals the next element
IEnumerable <Production <T> > productions = Productions.Where(a => a.NonTerminal.Equals(nextElement));
//for each of the possible following elements of item
foreach (Terminal <T> l in Follow(item, currentItems))
{
foreach (Production <T> p in productions)
{
//create a new item from the production
var newItem = new LRItem <T>(0, p);
//with a lookahead element of l from Follow(item)
newItem.LookaheadElement = l;
//if the item is not already contained
if (currentItems.Add(newItem))
{
//add the new item(but make sure it is not a duplicate
//items.Add(newItem);
//add the LR1 Closure of the new item
Lr1Closure(newItem, currentItems);
}
}
}
}
return(currentItems.Distinct());
}
/// <summary>
/// Gets a collection of Terminal elements that can appear after the element that is after the dot of the item.
/// </summary>
/// <example>
/// With Grammar:
/// S -> E
/// E -> T
/// E -> (E)
/// T -> n
/// T -> + T
/// T -> T + n
/// Follow(S -> •E) : {$}
/// Follow(T -> •+ T) : {'+', 'n'}
/// Follow(T -> •n, ')') : {')'}
/// </example>
/// <param name="nonTerminal"></param>
/// <returns></returns>
private IEnumerable <Terminal <T> > Follow(LRItem <T> item, IEnumerable <LRItem <T> > totalItems)
{
//Follow(item) is First(b) where item is:
//A -> a•Eb
GrammarElement <T> element = item.GetNextElement(1);
if (element != null)
{
if (element is NonTerminal <T> )
{
var firstSet = new List <Terminal <T> >();
//if the element has a production with no derived elements and the element is at the end of the current item's production, then
//add the lookahead of the given item.
//if there is any production of the current element that has no derived elements
if (Productions.Any(a => a.NonTerminal.Equals(element) && a.DerivedElements.Count == 0))
{
//if the current element is the end of the current item's production
if (item.GetNextElement(2) == null)
{
firstSet.Add(item.LookaheadElement ?? EndOfInputElement);
}
}
//select the lookahead element or end of input element for each item in the previous set
//List<Terminal<T>> firstSet = new List<Terminal<T>>(items.Select(a => a.LookaheadElement == null ? EndOfInputElement : a.LookaheadElement));
//add the rest of the first set.
firstSet.AddRange(First(element));
return(firstSet);
}
return(First(element));
}
if (item.LookaheadElement == null)
{
return new[] { EndOfInputElement }
}
;
return(new[] { item.LookaheadElement });
}
/// <summary>
/// Gets the collection of terminal elements that can appear as the first element
/// after the dot in the given LR Item. If next element, named T, is a terminal, then First(item) = {T}
/// </summary>
/// <example>
/// With Grammar:
/// S -> A
/// A -> A + B
/// A -> a
/// B -> b
/// First(S -> •A): {'a'}
/// First(A -> •A + B): {'a'}
/// First(A -> A • + B): {'+'}
/// First(B -> b): {'b'}
/// </example>
/// <param name="item"></param>
/// <returns></returns>
public IEnumerable <Terminal <T> > First(LRItem <T> item)
{
//get the next element and evaluate if it is a terminal or non terminal
GrammarElement <T> nextElement = item.GetNextElement();
//if nextElement is a Terminal then return {T}
if (nextElement is Terminal <T> )
{
return new[] { (Terminal <T>)nextElement }
}
;
//otherwise find all of the productions that have nextElement on the LHS.
Production <T> production = Productions.First(p => p.NonTerminal == item.LeftHandSide && p.DerivedElements.SequenceEqual(item.ProductionElements));
var firstElements = new List <Terminal <T> >();
if (production.DerivedElements.Count > 0)
{
if (!production.DerivedElements[item.DotIndex].Equals(production.NonTerminal))
{
GrammarElement <T> productionItem = production.GetElement(item.DotIndex);
if (productionItem != null)
{
//if it is a Terminal add to first elements
if (productionItem is Terminal <T> )
{
firstElements.Add((Terminal <T>)productionItem);
}
//otherwise add First(new LRItem(production)) of all of the productions where the LHS == productionItem
else
{
foreach (LRItem <T> i in Productions.Where(p => p.NonTerminal.Equals(productionItem)).Select(p => new LRItem <T>(0, p)))
{
firstElements.AddRange(First(i));
}
}
}
}
}
return(firstElements.Distinct());
}
/// <summary>
/// Gets a collection of LR(0) items that can be derived from the given item.
/// </summary>
/// <param name="item"></param>
/// <param name="currentItems"></param>
/// <returns></returns>
private IEnumerable <LRItem <T> > Closure(LRItem <T> item, IEnumerable <LRItem <T> > currentItems)
{
List <LRItem <T> > items;
items = new List <LRItem <T> >();
if (currentItems == null)
{
items.Add(item);
}
//if the next item is a non terminal, get all of the productions of that non terminal
if (item.GetNextElement() is NonTerminal <T> )
{
IEnumerable <Production <T> > productions = Productions.Where(a => a.NonTerminal.Equals(item.GetNextElement()));
//remove duplicate productions
if (currentItems != null)
{
productions = productions.Where(a => !currentItems.Any(i => i.LeftHandSide.Equals(a.NonTerminal)));
}
//add the productions as LRItems
items.AddRange(productions.Select(a => new LRItem <T>(0, a)));
//add the Closure of each found production
foreach (Production <T> p in productions)
{
if (p.DerivedElements[0] is NonTerminal <T> )
{
items.AddRange(Closure(new LRItem <T>(0, p), items));
}
}
}
return(items.Distinct());
}
/// <summary>
/// Gets a collection of LR(0) items that can be derived from the given item.
/// </summary>
/// <param name="item"></param>
/// <param name="currentItems"></param>
/// <returns></returns>
public IEnumerable <LRItem <T> > Closure(LRItem <T> item)
{
return(Closure(item, null));
}